As noted in Section 12.5-4 of the Handbook Of Separation Process Technology by R. W. Rousseau, Wiley Interscience (1987), thermal swing adsorption (TSA) is a common practice for the removal of trace or dilute impurities from liquid streams. The conventional TSA process consists of the following steps:
(a) flowing the contaminated liquid through a column packed with an adsorbent which selectively adsorbs the trace impurities and produces a pure product stream;
(b) draining the adsorption column to remove 40-50% of the entrained liquid from the column;
(c) heating the column to vaporize and desorb the remaining portion of the entrained liquid as well as the adsorbed impurities;
(d) cooling the column back down to the liquid feed temperature;
(e) filling the column with a portion of the purified liquid or feed liquid thereby making the column ready to start a new cycle beginning with step (a).
The above described process is usually adequate for purification of many liquid streams. It is commercially being used for removal of trace or dilute impurities (such as hydrocarbons) from water and drying of liquid hydrocarbons. There are two major practical problems with the above described process however as follows:
(1) When the liquid being purified is expensive, a very high recovery of this liquid is required. The problem is that a substantial portion of the product liquid may be lost during step (c) of the process. This is because the bulk liquid trapped in the voids of the solid adsorbent as well as that co-adsorbed with the impurity to be removed does not come out of the column during the drain step (b) but is instead vaporized and desorbed during step (c). Consequently, the column effluent during step (c) contains a large quantity of the component being purified along with the desorbed impurities. If the loss of the component being purified cannot be tolerated, a secondary separation process such as partial condensation or distillation is required to recover that component. That can be very expensive, particularly if the impurities form an azeotrope or a close boiling mixture with the product liquid.
(2) When the impurity to be removed is present in bulk quantities (greater than 2.0 mole %) and the quantity of the liquid to be purified is large, a very large volume of adsorbent is required. Since the cycle time of a conventional thermal swing adsorption process is controlled by the heating and cooling times (which are typically on the order of hours) a large volume of adsorbent is necessary to retain the large quantity of impurity present in the feed liquid mixture during the adsorption step. This leads to large losses of the desired product during the heating step as discussed in (1) above.